1. Field of the Invention
The present invention relates to a tone signal detector, a tone signal detection method, and a computer readable recording medium.
2. Description of the Related Art
In communication using an FM (Frequency Modulation) method or the like, as the technique for determining whether or not a call is established between a transmitter and a receiver, a technique using a tone signal is known.
Specifically, the transmitter superimposes a continuous tone signal of a single frequency on a modulated signal in advance and sends the resultant signal, and the receiver extracts the frequency component of the tone signal from the demodulated signal using a bandpass filter and then determines whether or not a call is established depending on whether or not the intensity of frequency component exceeds a predetermined value.
The receiver, when determining that a call has not been established yet, prevents noise from being output from a loudspeaker, for example, by turning off a relay prepared on a path from a demodulator to the loudspeaker.
Conventionally, various techniques of this kind for preventing an false detection of a tone signal have been disclosed.
For example, Japanese Utility Model Application KOKAI Publication No. S63-97928 (pp. 3-4, FIG. 1) discloses a tone squelch circuit, wherein the frequency of a tone signal is converted into a certain frequency and the converted signal is selected by means of a bandpass filter so that the tone signal of each frequency may be detected.
Moreover, Unexamined Japanese Patent Application KOKAI Publication No. H11-163748 proposes a technique for solving a problem that a receiver falsely detects noise as a tone signal, when the noise having a frequency component of the same frequency as that of the tone signal is superimposed on a modulated signal.
A tone signal detection circuit disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-163748 comprises: a first filter that extracts the frequency component of the frequency of the tone signal and extracts the frequency component in the vicinity of the frequency of the tone signal; a second filter that attenuates the frequency component of the frequency of the tone signal and extracts the frequency component in the vicinity of the frequency of the tone signal; and a subtractor.
Specifically, the first filter outputs the frequency component of the tone signal and the frequency component of noise, while the second filter outputs only the frequency component of the noise. The subtractor acquires and outputs only the frequency component of the tone signal by subtracting the frequency component which the second filter outputs from the frequency component which the first filter outputs.
However, with the techniques disclosed in both Patent Documents described above, the tone signal might be falsely detected or might not be detected when there is a difference in the amplitude of the tone signal at each frequency due to the communication conditions, such as a difference in the output of a transmitter or the radio wave environment.
Moreover, there is a strong desire to switch the configurations, criteria, or the like for detecting the tone signal depending on the situations and thereby prevent the false detection of the tone signal. However, the techniques disclosed in both Patent Documents described above do not employ a configuration for switching the configurations, criteria, or the like for detecting the tone signal depending on the situations.